The present invention relates to the art of impedance discrimination for accurately measuring small impedance differences and variations. The invention finds particular application in measuring capacitance variations such as may be caused by changes in pressure. It is to be appreciated, however, that the invention may also be applicable to measuring other impedance variations and other physical phenomenon which cause such impedance variations including temperature, force, flow, acceleration, position, chemical concentration, and the like.
Heretofore, capacitive sensors have been incorporated in transducers for measuring the above listed and other physical phenomenon. An interface circuit monitors variations in the capacitance and produces an output signal which varies in accordance therewith. In one conventional interface circuit configuration, the capacitive element controls the frequency of an oscillator. The output frequency then changes in proportion to the changes in capacitance, hence, the sensed physical condition. However, variable frequency interface circuits commonly provide relatively poor baseline stability. Changes in the frequency are also attributable to time, temperature, and other events in addition to the sensed physical condition. Moreover, the accuracy and speed with which the capacitance changes can be measured are limited by the accuracy and speed which changes in the frequency can be resolved.
Another commonly used interface circuit includes a ratio detector such as a diode bridge to detect the change in capacitance relative to a null point. One of the drawbacks of the ratio detector is that its sensitivity is relatively poor. Small variations in capacitance are difficult to differentiate particularly where stray capacitances may be large relative to the capacitance of the sensor. This is a common problem with silicon-to-silicon capacitance sensors. Furthermore, because variations in the stray capacitance vary the null point of an individual transducer chip, on-chip trimming is required.
In a conventional calibration procedure, the sensitivity and the null point of the transducer are commonly adjusted. Both the variable frequency and the ratio detector interface circuits have interdependent sensitivity and null point adjustments. That is, adjusting one of the sensitivity and null point also alters the other. This renders calibration to a preselected operating point difficult and, in some instances, impossible. Further, this interdependent adjustment inhibits the incorporation of micro-controller or other automatic calibration systems.
The present invention contemplates a new and improved impedance measurement circuit which overcomes the above referenced problems and others.